1. Field of the Invention
The present invention relates to a power amplifier, such as GaAs HBT or Si bipolar transistor power amplifier, and more particularly to a protection circuit for preventing the breakdown of an output-stage transistor of a power amplifier during load change (during output load mismatch) in output overvoltage conditions.
2. Description of the Related Art
In power amplifiers for mobile communication, a GaAs MESFET, GaAs HEMT and GaAs-based HBT are expected as future power elements for mobile communication, by reason that they have the following advantages as compared to a conventional FET:
(1) Capability of single power-supply operation because of no need for negative gate bias voltage;
(2) Capability of ON/OFF operation without any analog switch on drain side as with a Si-MOSFET; and
(3) Capability of providing high output current density and facilitating the reduction in size required for obtaining a desired output as compared to a FET power amplifier.
By utilizing these features, a HBT (Heterojunction Bipolar Transistor) is becoming increasingly widely used in 2W–4W high power portable telephones, such as European GSM (Global System for Mobile Communication: currently most widely used 900 MHz band portable telephone system), in place of a Si-MOSFET which has been primarily used therein.
In the GSM application, a power amplifier is used in such a manner that a power supply terminal thereof is connected directly to a battery power supply without interposition of any voltage regulator. In addition, for the purpose of downsizing, any isolator for suppressing the variation in load impedance of the power amplifier due to fluctuation in output impedance from an antenna terminal is not used between an output terminal of the power amplifier and the antenna end (portable telephones (PDC: Personal Digital Cellular) in Japan generally employ an isolator).
Consequently, if a load impedance is changed to a value far greater than a usual value of 50 Ω when a power supply voltage is increased to a value (e.g. 4.5 V to 5.5 V) greater than a recommended operating condition (3 V to 3.6 V) during battery charge, a load curve of a final-stage transistor will be significantly changed, resulting in thermal breakdown of the final-stage transistor.
Generally, attempts to obtain enhanced current gain or reduced parasitic resistance/capacitance in order to improve transistor characteristics at a low voltage (nominal operating voltage: 3V to 3.6 V) in a portable telephone to be designed for low-voltage operation are apt to cause deterioration in tolerance on Vce (safe operation range). Therefore, portable telephone systems, such as GSM, where the fluctuation or change in power supply voltage or load has direct impact on a power amplifier have seriously suffered from the aforementioned problem of transistor breakdown.
There has been known a technique of detecting an excessive current flowing through the base of a final-stage amplifier HBT, and cancelling out the excessive current to prevent a collect current from increasing (see, for example, the following Patent Publication 1).
There have also been known a technique of turning off an output transistor when an output current detected at the output transistor is increased up to an overcurrent limit (see, for example, the following Patent Publication 2), and a technique of supplying a feedback current to the base of a final-stage transistor to control the voltage at the collector of the transistor when a voltage equal to or greater than a given value is applied to the collector (see, for example, the following Patent Publication 3).
[Patent Publication 1]
Japanese Patent Publication No. 2002-76791 titled “Power Amplifier Module” (claim 1 and FIG. 1)
[Patent Publication 2]
Japanese Patent Publication No. 2003-78362 titled “Power Semiconductor Device” (paragraph [0020] and FIG. 1)
[Patent Publication 3]
Japanese Patent Publication No. 2000-341052 titled “Power Amplifier Protection Circuit” (claim 1 and FIG. 1)
All of the techniques disclosed in the above Patent Publications are designed to detect a collector or base current to a transistor. Thus, these techniques cannot prevent thermal breakdown of the transistor due to over output of the transistor itself